A cold accumulation refrigerator such as a Gifford-McMahon (GM) refrigerator, a pulse tube refrigerator, etc., can generate the cold in a range from a low temperature of about 100 K to a cryogenic temperature of 4 K (Kelvin), and can be used for cooling a superconducting magnet, a detector or the like or in a cryopump, etc.
For example, according to the GM refrigerator, working gas such as helium gas compressed by a compressor is introduced to a regenerator where the working gas is cooled by a regenerator material in advance. Further, the working gas generates the cold according to expansion work in an expansion chamber, and passes through the regenerator again to return to the compressor. At that time, since the working gas is to be introduced again, it passes through the regenerator while cooling the regenerator material. This process is one cycle and the cold is thus generated periodically.
According to such a cold accumulation refrigerator, if it is necessary to generate the cryogenic temperature lower than 30 K, a magnetic material such as HoCu2 is used as a regenerator material of the regenerator.
Recently, the use of helium gas as the regenerator material of the regenerator is contemplated. Such a regenerator is also referred to as a helium-cooling type regenerator. For example, US Patent Publication No. 2006/0201163 discloses using, as the regenerator material of the regenerator, plural capsules with thermal conductivity which are filled with helium gas.
FIG. 1 illustrates change characteristics of specific heat vs. temperature with respect to the helium gas and the HoCu2 magnetic material. As is apparent from FIG. 1, in a cryogenic temperature range around about 10 K, the specific heat of the helium gas at a pressure of about 1.5 MPa exceeds the specific heat of the HoCu2 magnetic material. Thus, in such a temperature range, more effective heat exchange can be performed by using the helium gas instead of using the HoCu2 magnetic material.